It is well known that when recording closely spaced data bits (such as dibits) on a nonlinear communication medium (for example, writing to a magnetic disk or an optical disk) the position of the bits may be shifted. This shift (which is called nonlinear bitshift) degrades the performance of known data detection schemes such as the PRML (partial-response-maximum-likelihood) detection scheme, since these detection schemes are based on the assumption of a linear channel response.
It is possible to compensate the nonlinear bitshift effect by shifting the data bit positions during data writing. However, to design a compensation scheme, the magnitude of the nonlinear bitshift must be known. Unfortunately, measuring the nonlinear bitshift has proven difficult; the prior art has accomplished such measurement only by means of sophisticated and expensive equipment.
Several methods requiring such equipment have been proposed for measuring nonlinear bitshift. One is to compare the data bit response with isolated transitions to identify the nonlinear bitshift. A second uses a specially designed pseudo-random sequence to determine an impulse response. The nonlinear bitshift is then measured as an echo at a known position. A third uses a Volterra modelling technique for characterizing nonlinear distortion in magnetic recording channels. All these methods require complicated data manipulation and high precision waveform recording. Thus, all are complex and costly when used to provide routine measurement of nonlinear bitshift such as that required in a manufacturing or operational environment.